The present invention relates to thermoluminescent phosphors used in the detection of radiation. In particular, it pertains to LiF phosphors doped with magnesium intended for use in mixed radiation fields including, e.g. low energy beta radiation emitted by tritium in the presence of more penetrating X and gamma radiation. With the addition of a hydrogenous overlay such phosphors also can be used to detect fast neutrons that produce shallowly penetrating recoil protons as they pass through the hydrogenous layer.
Phosphors of LiF have been widely used in the measurement of radiation in clinical applications and personnel dosimetry. Such phosphors are commercially available, for instance the TLD-100 manufactured by the Harshaw Chemical Company, Solon, Ohio. This phosphor contains lithium in its natural isotopic ratio (92.5% Li-7 and 7.5% Li-6 by weight), approximately 100-200 parts per million magnesium and about 15 parts per million titanium by weight.
Phosphors of this type, on exposure to radiation will store energy that subsequently can be released at a convenient time as thermoluminescence by heating to appropriate temperatures. The thermoluminescence can be recorded to provide a glow curve with peaks at characteristic temperatures. FIG. 1 illustrates such a glow curve for a TLD 100 phosphor after exposure to CS-137 gamma rays and to electrons. The glow curves include a number of thermoluminescent peaks between room temperature and about 200.degree. C. Peaks at higher temperatures are seldom observed with this phosphor.
The present inventors have observed and reported a thermoluminescent peak at about 290.degree. C. from a TLD-100 phosphor ("TL Response to 1-30 KeV Electrons of TLD-100 and TLD-100 Diffused with Various Elements," Technical Report, C00-1105-226, University of Wisconsin, 1975). This publication suggests that a LiF phosphor doped with an unknown impurity in an outer layer can be employed to discriminate between radiation which deposits its energy in the outer layer and other radiation which deposits its energy within the bulk of the phosphor. However, at the time of this publication, the critical dopant responsible for this new peak was unknown.
One of the previous methods used for personnel monitoring of tritium in a mixed radiation field involves stacking of two TLD-100 thermoluminescent chips. The beta radiation from tritium penetrates only the top of the first chip. However, X-rays and gamma rays will penetrate both chips, thus allowing some discrimination between the two types of radiation. Unfortunately, tritium beta rays penetrate only a few micrometers while the thinnest LiF chips practical for routine use are about 1 mm in thickness. This means that the thermoluminescence resulting from tritium exposure in the top chip is diluted by more than a factor of 100, when compared to the reading resulting from X-ray or gamma ray exposure of the second chip.
Therefore, in view of the disadvantages of the prior art, it is an object of the present invention to provide a new thermoluminescent phosphor including LiF in major portion that can be used to detect shallowly penetrating radiation in the presence of a background of more penetrating radiation.
It is a further object to provide a LiF, thermoluminescent phosphor having suitable dopants for providing a thermoluminescent peak at about 290.degree. C.
It is also an object of the present invention to provide a LiF thermoluminescent phosphor with a dopant only within the outer layer thereof which will permit discrimination between radiation that deposits its energy in the outer layer from radiation that deposits its energy within the bulk of the phosphor.
It is another object of the present invention to provide a method of preparing a LiF thermoluminescent phosphor containing a particular dopant in portions thereof which will produce recognizable thermoluminescent peaks only from that doped portion.